Chlorofluorocarbons (CFC's) are widely used solvents for precision cleaning of parts and components due to their advantageous physical and chemical properties, especially their solvency for contaminating materials such as oils, greases, resins, fluxes, particulates, and other contaminates. Examples of such solvents commonly used in many applications are CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane) and CFC-11 (trichlorofluoromethane). These solvents are used to clean and/or degrease components or systems related to, but not limited to, oxygen handling systems, refrigeration equipment, heat pumps, electronics, implantable prosthetic devices, and optical equipment.
A refrigeration or air conditioning system, for example, may have drastically reduced performance resulting from compressor failure caused by retained contaminants. Such systems require periodic flushing to remove contaminants such as oil, water, acid and sludge. The need to properly clean these contaminated systems is very important and trichlorofluoromethane (CFC-11) has been found to be an effective and versatile solvent. Being able to dissolve an unusually large array of contaminants and having excellent physical characteristics, CFC-11 became the ‘solvent-of-choice’ for refrigeration flushing and its use spread to other applications.
CFC-113 and CFC-11 have also been used to measure residue remaining in a system. For example, in Air Force launch vehicle applications involving liquid or gaseous oxygen systems where residual contamination can be catastrophic, CFC-113 has been used to detect and quantify the amount of hydrocarbon and fluorocarbon residues. Both of these chlorofluorocarbons are also commonly-used for foam blowing and polymer coating.
CFC-113 and CFC-11 have many favorable characteristics such as low toxicity, non-flammability and stability. Furthermore, they are not classified as air-polluting volatile organic compounds (VOC's) by environmental regulators and have a high worker exposure threshold value, thus eliminating the need for costly air circulation or dilution precautions. Due to concerns over worker safety from toxic chemical exposure and hazardous waste disposal resulting from the use of VOC's, these desirable characteristics led to the widespread use of CFC-113 and CFC-11.
However, by the mid 1980s, problems relating to the ability of certain halogenated hydrocarbons to react with and deplete atmospheric ozone became apparent. As a result, the use of CFC-113 and CFC-11 was restricted under the Montreal Protocol. In 1987, twenty-four nations agreed in principle to control ozone-depleting substances (ODS). Although CFC solvents had become critical in industry, the importance of protecting the earth's ozone layer weighed heavier. Thus, non-toxic and non-ozone depleting replacement solvents became a priority for refrigeration technicians, electronics manufacturers, and the military. Various CFC substitutes have emerged and often rely on solvents such as n-propyl bromide and dichloroethylene, which are flammable and not as desirable as CFC-113 or CFC-11.
Because CFC-113 and CFC-11 possess so many desirable properties, those skilled in the art have attempted to find replacements with limited success, most believing that a replacement solvent must compromise on some performance properties.
Many factors are important when selecting CFC replacement solvents. Some of the performance properties for a CFC replacement include cleaning effectiveness or solvency, volatility (e.g., boiling point), compatibility with materials to be cleaned (e.g. metals, elastomers and systems), toxicity (e.g., LC50, LD50, cardiac sensitization, mutagenicity, skin irritation), environmental persistence (e.g., ozone depletion potential (ODP), global warming potential (GWP), biodegradability, flammability (flash point), cost and availability.
Hazardous risks such as toxicity, environmental impact and flammability are important since the replacements will likely be used in large volumes as manufacturers transition away from CFC-113 and CFC-11. The hazard potential of the candidate replacements can be characterized using toxicity information such as lethal doses (LD), lethal concentrations (LC) or threshold limit values (TLV), and flammability information. Environmental properties can be analyzed through ozone depletion potential (ODP) and global warming potential (GWP). Volatility can be assessed using the normal boiling point (nBP) of the solvent. For a discussion of toxicity and environmental parameters, see e.g., U.S. Pat. No. 6,300,378. The following paragraphs discuss the relevance of these performance parameters.
Cleaning Effectiveness or Solvency
The cleaning effectiveness of CFC-11 is unique in that it is able to dissolve and absorb an array of different materials like oils, greases and acids. The solvency of the replacement should be comparable to CFCs so that this primary metric of performance is not compromised.
Volatility
The volatility of a replacement solvent can be measured in terms of its normal boiling point (nBP). The volatility of the replacement solvent should be similar to CFCs so there is minimal impact on existing cleaning systems by switching solvents. For example, an effective solvent should be volatile enough to evaporate, but should not flash off of surfaces since the solvent preferably remains in contact with contaminants long enough to dissolve them.
Compatibility
Material and system compatibility is another desired property for a replacement solvent. The solvent is preferably compatible with metals such as aluminum, copper, carbon steel and stainless steel, as well as elastomers. The solvent should not degrade or corrode surfaces in the system being cleaned.
Toxicity
Parameters such as the lethal dose 50 (LD50), lethal concentration 50 (LC50), cardiac sensitization, skin irritation, and mutagenicity (e.g., via the Ames test) can be used as toxicity metrics. The LDn or LCn abbreviations, where n is the percent lethality, are used for the dose of a toxicant lethal to n % of a test population. For example, at LD50, 50% of the recipients of that particular toxic dose would die. Cardiac sensitization is a measure of the ability of a compound to cause cardiac arrhythmia under stress. Generally, it is desired to minimize these parameters and select compounds that have lower values than the solvent that is being replaced.
Environmental Persistence
The environmental persistence of a solvent is also very important. Parameters such as the ozone depletion potential (ODP) and global warming potential (GWP) are measures of this attribute. ODP and GWP give the relative ability by weight of a chemical to deplete stratospheric ozone and to contribute to global warming, respectively. Values for ODP and GWP are calculated based on an earth surface release and then reported relative to a reference compound (typically CFC-11 for ODP and CFC-11 or carbon dioxide for GWP). Generally, the ODP should be less than 0.02, and the GWP should be minimized, preferably lower than the solvent being replaced.
The biochemical oxygen demand (BOD) is another measure of persistence typically in groundwater, lakes, and other bodies of water.
Flammability: Flashpoint
Whether a solvent is suitable as a cleaning solvent is partially dependent upon its flammability, which can be quantified by the flashpoint of the solvent. The flashpoint is the temperature at which a liquid gives off vapor sufficient to form an ignitable mixture with air (oxygen) near the surface of the liquid. The ideal replacement refrigeration solvent should have a flashpoint greater than about 40° C. This categorizes the solvent as non-flammable and insures a wide range of conditions whereby the solvent can be used safely.
CFC-113 and CFC-11 replacements and solvents that address ozone depletion have been introduced and are disclosed, for example, in U.S. Pat. Nos. 5,035,828, 6,402,857, 6,297,308, and 6,020,298. Various solvents and solvent mixtures are disclosed which have low ODPs. These replacement solvents, however, do not possess all of the desired properties of CFC-11 or CFC-113, such as cleaning effectiveness, oxygen compatibility, toxicity and flammability.
In U.S. Pat. No. 5,035,828, HCFC-234 is combined with an aliphatic alcohol or cyclohexane, but this mixture is easily flammable. U.S. Pat. No. 6,402,857 utilizes n-propyl bromide with other organic constituents, which are also flammable and have a significant adverse impact on ozone. U.S. Pat. No. 6,020,298 utilizes hydrofluoropolyethers, and U.S. Pat. No. 6,297,308 utilizes halogenated ethers and hydrocarbons with a surfactant. While these solvents appear to avoid damage to the ozone layer, the perfluorinated compounds contained therein are known to be potent greenhouse gases.
Solvents that meet some environmental restrictions and are non-flammable are disclosed in U.S. Pat. Nos. 6,300,378 and 5,759,430 and in Tapscott & Mather, “Tropodegradable Fluorocarbon Replacements for Ozone-Depleting and Global-Warming Chemicals,” J. Fluorine Chemistry 101:209-213 (2000). The compounds disclosed therein are generally non-ozone depleting and/or non-flammable, as they are “tropodegradable fluorocarbons,” which are defined as compounds having structural weaknesses to ensure rapid decay in the troposphere.
When tropodegradable fluorocarbons are exposed to sunlight or chemical radicals (e.g., hydroxyls) in the atmosphere, they decay into forms that do not damage the ozone layer or contribute to the greenhouse effect. This structural weaknesses can take such forms as hydrogen being present in the molecule, a vulnerable carbon-carbon double bond, an ether bond, or a bromine atom being present for easy degradation. These structural vulnerabilities render the molecules unstable, and within a fairly short period of time they break down and are no longer part of the atmosphere. The foregoing references, however, fail to teach solvents with optimized solvency, together with desirable toxicity, and material compatibility.
Additional solvents are disclosed in U.S. Pat. Nos. 6,291,417, 5,273,592, 5,174,906, and 4,999,127. Commonly-owned U.S. Patent Application Publication No. 2006/0166851 discloses a list of replacement solvents for CFC-113.
Commercially-available products that are used as refrigeration flushes and claim to be replacements for CFC-11 include RX-11 and Supercool. RX-11 is marketed by Nu-Calgon. Supercool is used in automotive air conditioning units.
One object of the present invention is to provide CFC solvent replacements preferably comprising at least two tropodegradable components that act collectively to provide solvent mixtures that have improved cleaning effectiveness or solvency with respect to the CFC targeted for replacement, boiling points greater than about 40° C., compatibility with common elastomers and metals, toxicities less than or similar to the CFC targeted for replacement, ODP values less than about 0.02, and are non-flammable as measured by flashpoint testing.
Regarding the requirement for cleaning effectiveness/solvency, the inventive solvent mixture advantageously can absorb impurities contained in a refrigeration system including oil, acid and moisture. According to a preferred embodiment, the solvent mixture includes at least one alcohol, which is an effective water absorber. In this regard, the smaller chained alcohols such as ethanol and propanol are preferred because they absorb the most moisture per unit volume. The larger chained alcohols on the other hand, although less efficient at trapping water, are less flammable than the short chained alcohols.
Because contaminated oil is generally the major impurity that is to be flushed from system lines, the solvent mixture preferably comprises a hydrocarbon as a main component. Oil, which is also a hydrocarbon, is most readily dissolved by a hydrocarbon-containing solvent mixture.
Regarding flammability, the flash point of the solvent mixture is preferably greater than about 40° C. (100° F.) in order for the solvent mixture to be classified non-flammable and be considered safe to use in refrigeration systems.
Yet another object of this invention is to provide replacement solvent mixtures to clean and/or degrease components or systems related to, but not limited to, refrigeration systems, heat pumps and air conditioning units.
Non-flammable solvent mixtures and/or solvent mixtures having reduced flammability can be prepared by adding a property-modification solvent selected from Table 1 to a main solvent. Also, solvent mixtures having improved acid and water absorption properties can be prepared by adding a property-modification solvent selected from Table 1 to a main solvent such as alcohols, glycols, esters or ketones.